The present invention relates to an antenna switching apparatus for switching and selecting a diversity antenna of good reception from a plurality of antennas properly.
The present application claims priority from Japanese Application No. 2001-244732, the disclosure of which is incorporated herein by reference for all purposes.
Conventionally, there has been known a diversity receiving system which has a facility to automatically switch and select a diversity antenna of good reception among a plurality of antennas so as to ensure favorable reception performance in receiving radio broadcasts such as an FM broadcast.
As shown in FIG. 9, this diversity receiving system has a switching circuit 1 which is interposed between a plurality n of antennas ANT1-ANTn and an intermediate frequency signal processing unit 2. The switching circuit 1 switches and selects a single antenna exclusively so that a high-frequency reception signal Sin occurring in the antenna switched and selected is introduced to the intermediate frequency signal processing unit 2 for so-called down conversion.
Then, a detector 3 detects an intermediate frequency signal SIF output from the intermediate frequency signal processing unit 2, thereby generating a detected signal Sdt. This detected signal Sdt is supplied to an output circuit 4 which is equipped with an audio amplifier or the like, so that a reproduced signal Sout is output.
In order to detect an antenna of good reception, there are also provided a noise detecting filter 5, a comparator 6, and a control unit 7. The noise detecting filter 5 passes a noise component Sn contained in the detected signal Sdt.
The comparator 6 detects the level of the noise component Sn based on a predetermined threshold THD, and outputs a noise detection signal Sc. The noise detection signal Sc has a logical value of “H” when the noise component Sn is higher than the threshold THD in level, and a logical value of “L” when the noise component Sn is lower than the threshold THD in level.
The control unit 7 counts the number of times N when the noise detection signal Sc shows the logical value “H” within a predetermined time Tns elected in advance. When the number of times N falls below a predetermined count Nns, the control unit 7 determines that the antenna selected currently is of good reception. When the number of times N exceeds the predetermined count Nns, it determines that the antenna selected currently is of poor reception. If the antenna selected currently is determined to be of poor reception, a switching control signal Sv is output to control the switching circuit 1 to switch to another antenna.
The control unit 7 also counts the number of times M of antenna switching within a predetermined time Tch which is set to be longer than the foregoing time Tns. If the number of times M exceeds a predetermined count Mch, the control unit 7 makes the switching circuit 1 suspend antenna switching, so that a certain fixed antenna is used for reception.
More specifically, reception is performed with a certain fixed antenna so as to suppress useless antenna switching, provided that if antenna switching comes to occur frequently, switching to other antennas does not necessarily select an antenna of the best reception.
As stated above, the foregoing conventional diversity receiving system uses a certain fixed antenna for reception in situations where antenna switching comes to occur frequently. There has been a problem, however, that in making “the determination whether or not antenna switching is occurring frequently”, a reduction in the time Tch for counting the number of times M of antenna switching can cause more frequent fixing of antennas, making the diversity effect smaller.
On the other hand, if the time Tch for counting the number of times M of antenna switching is extended, it becomes difficult to determine the quality of reception precisely. This has produced a problem of frequent control errors in antenna switching.
Moreover, the comparator 6 compares the predetermined threshold THD and the noise component Sn to determine reception quality, whereas simply comparisons between the threshold THD and the noise component Sn do not promise high-precision determinations on reception quality. This has caused a problem of difficulty in suppressing control errors in antenna switching. In other words, there has been the problem that simply comparing the threshold THD and the noise component Sn cannot achieve high-precision determinations on reception quality under momentary variations in the field strength of incoming waves, the influence of surrounding environment on the antennas, and so on.